It's weird there are no Nikon "L" lenses you just have to judge how good they are by the cost or buy a macro lens as they tend to be the best quite often. I think the Sigma lenses are just as sharp the main difference is the build quality and solidity. It varies though, larger lenses tended to be more solid just by way of having more glass and even the main brand lenses these days are very plastic compared to vintage lenses which were made of metal.
I only found out after buying some second hand lenses how much of a difference it makes. Some of those old lenses are pretty indestructible.